Microwave attenuator



Jan. 21, 1964 R. A. JORDAN 3,119,081

MICROWAVE ATTENUATOR Filed Sept. 25, 1961 2 Sheets-Sheet 1 FIG.|

INVENTOR. ROBERTA. JORDAN Jan. 21, 1964 R. A. JORDAN 7 3,119,081

MICROWAVE ATTENUATOR Filed Sept. 25, 1961 2 Sheets-Sheet 2 INVENTOR.

ROBERT A. JORDAN '7 AT ORNEY United States Patent 3,119,081 MICROWAVEATTENUATOR Robert A. Jordan, Essex, Mass., assiguor to Laboratory forElectronics, Inc., Boston, Mass, a corporation of Delaware Filed Sept.25, 1961, Ser. No. 140,467 1 Claim. (Cl. 333-81) This invention pertainsgenerally to microwave transmission circuits and particularly to animproved attenuator adapted to insertion in a microwave transmissiononcuit of the coaxial cable type.

It is known that selective attenuation of microwave energy is one toadjust the power level of such energy n the individual branches of abranched microwave circuit, or to or from a microwave component.Particular care must be taken, however, to avoid unwanted andundesirable effects. In particular it must be recognized that asatisfactory attenuator must not introduce undesired effects, as a phaseshift dependent upon attenuation. Further, a satisfactory attenuatorshould be easy to manufacture and simple, yet dependable, in operation.Additionally, there is a great advantage to be gained if an attenuatormay be continuously varied over a range of attenuation.

Therefore, it is a primary object of this invention to provide, for usein a microwave transmission circuit, an improved attenuator which may befabricated using printed circuit techniques.

Another object of this invention is to provide an unproved, continuouslyadjustable microwave attenuator which has almost no effect on microwaveenergy except to change its power level.

Still another object of the invention is to provide an improvedmicrowave attenuator which is simple and dependable in use.

These and other objects of the invention are attained generally byproviding a photo-etched microwave attenuator in which a power absorbingelement is movably supported in contact with a shaped conductor, suchconductor being inserted in circuit with the center conductors of twosections of coaxial cable. Means are provided, also, to shield the powerabsorbing element and the shaped conductor so that the electricalcharacteristics of the sec tions of transmission line are unaffected,except as desired, by the attenuator.

For a more complete understanding of the invention, reference is nowmade to the following description of a preferred embodiment of theinvention, and to the drawings, in which:

FIG. 1 is a perspective view of a microwave attenuator according to theinvention; and,

FIG. 2 is an exploded view of the assembly shown in FIG. 1 to illustratein detail the shape and disposition of the various elements making upthat assembly.

A microwave signal entering the illustrated device on, say coaxial cable11, may have its power level attenuated by any desired amount from zeroto 25 to 40 db before leaving the device via a second coaxial cable 13.Obviously then, if the free end of coaxial cable 11 is connected to oneof the output terminals of a microwave signal source (not shown), as,for example, a power divider, and if the free end of the coaxial cable13 is connected to a circuit element (again not shown), as a drivenelement in an antenna array, the power level of the microwave signal atsuch a driven element with respect to the power level of the signal atthe power divider may be adjusted as desired.

Any desired attenuation is here accomplished by shaping the variousportions of the center conductor of the attenuator as illustrated, andat the same time, providing means for moving one such portion relativeto another.

3,119,081 Patented Jan. 21, 1964 Thus, the center conductor of theillustrated attenuator is made up of length of an electrical conductor,preferably photo-etched and having parts indicated by the numerals 15,17, 23 disposed, respectively, on a plate 25 and a plate 27 (all ofwhich last named parts are electrical insulators) and means, describedhereinafter, for rotating a disk 29 so as to move a semicircularattenuating section 30 with respect to lengths 17. It will be noted thatwhile any known electrically insulating material may be used as the basematerial for plate 25, plate 27 and disk 29, it has been found thatRexolite, a material manufactured by the Rex Corporation of Acton,Massachusetts, is suited for the purpose, since the mechanical andelectrical characteristics of Rexolite are quite well adapted to theirillustrated application. It will also be noted that the various lengthsof electrical conductors (as Well as the other electrically conductiveportions of the attenuator presently to be described) may be copper,silver, aluminum or any other electrical conductive material and thatthe dimensions of the various elements, as the width and thicknes of thelength of conductors 15, 17, 23 and the thickness of the plates 25, 27are matters of design. Fur ther, it should be noted that thesemicircular attenuating section 30 may be any power absorbing materialalthough, in the illustrated case, a sheet of so-called Filmed Mylar(consisting of a base of the plastic known in the art as Mylar, aproduct of E. I. du Pont de Nemours & Co., and a film of carbon electrodeposited thereon) has been used. That is, the particular dimensions ofthe various elements may be varied, as is well known, to adapt theillustrated attenuator to use with transmission lines of differingcharacteristic impedances at dilfering frequencies. It is important onlythat length 17 be shaped so as to have a semicircular section 17a andthat semicircular attenuating section 30 overlap the semicircularsection 17a when disk 29 is inserted in a complementary opening 31 inplate 25.

The electrical circuit through the attenuator is completed by providinga ground plane 33 on the plate 25 and a ground plane 35 on the plate 27and an opening (not numbered) formed through ground plane 35 and theplate 27 to allow the center conductors of coaxial cables 11, 13 toproject into contact, respectively with lengths 15, 23. Ground plane 33and ground plane 35 are, of course, metallic, preferably being depositedas a film on plates 25, 27 and photo-etched as indicated. The two planesfurther are electrically connected to each other and to the shields ofcoaxial cables 11, 13, as by a number of screws 37 (which also serve tohold the coaxial cables 11, 13 in place) in appropriately disposedopenings through both plates 25, 27. It is clear from the foregoing,therefore, that a complete electrical circuit is formed between both theshields and the center conductors of the coaxial cables 11, 13 when theelements are assembled into the unitary structure shown in FIG. 1.

Turning now to a description of the means for rotating disk 29 referredto hereinbefore, it may be seen that the disk 29 is attached, as byriveting to a metallic spur gear 39 having a root diameter greater thanthe diameter of opening 31. A shaft 41 is afiixed in any known mannercentrally of the spur gear 39 and a helical spring 43 is fitted over theshaft 41. Thus, when the shaft 41 is fitted into a journal bearing 45 ina bridle 47 and the bridle 47 is mounted, as with screws, to the plates25, 27, the disk 29 is springily pressed into the opening 31, curvedsections 17 being then forced into contact with semicircular attenuatingsection 30. At the same time, the peripheral portion of the side of thespur gear 39 adjacent to the disk 29 is pressed into contact with theunderlying portions of the ground plane 33. A driving gear 49, meshingwith spur gear 39 and supported on a shaft 51 fitted into a journalbearing 53 in the bridle 47 permits a Vernier 3 adjustment of the spurgear 39, and the parts carried thereby, to be made.

In practice, it is highly desirable that positive means be provided tolimit rotation of the semicircular attenuating section 30. This issimply accomplished here by mounting stops 55 on the plate 25 and acooperating arm 57 on the spur gear 39. Rotation of the spur gear,obviously, is then restricted to the sector between the stops 55 so thatmovement of the semicircular attenuating section 30 is limited.

It will be immediately obvious to those having skill in the art thatmany changes in the just described attenuator may be made withoutdeparting from the concept of the invention. For example, it may onoccasion be desirable to provide a non-linear attenuator. This may beeasily accomplished by merely replacing the contemplated linear powerabsorbing sheet with a sheet having a non-linear coating of carbon.

In view of the foregoing description of a preferred embodiment of theinvention and of the obvious modifications which may be made thereinwithout departing from the inventive concepts so disclosed, it is feltthat the invention should be limited only by the spirit and scope of theappended claims.

What is claimed is:

An attenuator for use in a microwave transmission line including twocoaxial cables, comprising: a first electrically insulating plate havinga first electrically conductive coating deposited on its outside surfaceand a semicircular section of electrically conductive line deposited onits inside surface; means operative through the first electricallyconductive coating and the first electrically insulating plate toconnect the center conductor of a coaxial cable to the ends of thesemicircular section of line; a second electrically insulating platehaving a circular opening formed therethrough and an electricallyconductive coating deposited on the outside surface thercof; anelectrically insulating disk having a diameter less than the diameter ofthe circular opening and a semicircular sheet of microwave powerabsorbing material aifixed to one side thereof; an electricallyconductive gear aflixed to the opopsite side of the electricallyinsulating disk, the root diameter of such gear being greater than thediameter of the circular opening; and means for mounting the first andthe second insulating plate, the electrically insulating disk and theelectrically conductive gear to establish a first movable electricalcontact between the semicircular section of line and the semicircularsheet of microwave power absorbing material and a second movable contactbetween the peripheral portions of the electrically conductive gear andthe there underlying portions of the electrically conductive coating onthe second electrically insulating plate.

References Cited in the file of this patent UNITED STATES PATENTS2,909,736 Sommers et al Oct. 20, 1959 2,961,620 Sommers Nov. 22, 19602,961,621 Tannenbaum et al Nov. 22, 1960 2,961,622 Sommers Nov. 22, 19603,046,505 Wilson July 24, 1962

